Furnace for sintering powder

ABSTRACT

Sintered bodies are formed from powder material by sintering a body produced from a powder at atmospheric pressure or below and at such a temperature that the powder grains adhere to each other and thereafter isostatically hot pressing the sintered bodies to high density in a furnace containing an inert atmosphere. The powder may contain hard metal particles such as metal carbides and a binder of an iron group metal. The furnace is formed by a cylindrical pressure chamber with a high pressure cylinder and end closures projecting into the cylinder. A furnace chamber is arranged in the pressure chamber and has heating windings and an insulating lid and bottom. One end closure of the pressure chamber has an annular part with members sealing against the cylinder and a valve plate covering the opening of the annular part. A vacuum conduit is provided which can be sealingly connected to the annular part and which includes projecting members for separating the valve plate and the annular part when the conduit is placed in operative position.

United States Patent Isaksson [54] FURNACE FOR SINTERING POWDER [72] Inventor: Sven-Erik lsaksson, Robertsfors,

Sweden [73] Assignee: Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden [22] Filed: July 9, 1971 [21] Appl.No.: 161,200

Related U.S. Application Data [62] Division of Ser. No. 12,701, Feb. 19, 1970,

abandoned.

30 Foreign Application Priority om March 3, 1969 Sweden.. 2856/6 9 [52] U.S. Cl. ..266/24, 266/5 R [51] int. Cl. ..C2ld 1/00 [58] Field of Search .,...266/24, 5 R, 2.5; 75/226 [56] References Cited UNITED STATES PATENTS 3,571,850 3/1971 Poltoetal. ..75/226 [451 Nov. 21, 1972 Primary Examiner-Gerald A. Dost Attorney-Jennings Bailey, Jr.

[57] ABSTRACT Sintered bodies are formed from powder material by sintering a body produced from a powder at atmospheric pressure or below and at such a temperature that the powder grains adhere to each other and thereafter isostatically hot pressing the sintered bodies to high density in a furnace containing an inert atmosphere. The powder may contain hard metal particles such as metal carbides and a binder of an iron group metal. The furnace is formed by a cylindrical pressure chamber with a high pressure cylinder and end closures projecting into the cylinder. A furnace chamber is arranged in the pressure chamber and has v heating windings and an insulating lid and bottom.

One end closure of the pressure chamber has an annular part with members sealing against the cylinder and a valve plate covering the opening of the annular part. A vacuum conduit is provided which can be sealingly connected to the annular part and which includes projecting members for separating the valve plate and the annular part when the conduit is placed in operative position. i

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svEN-Eiem \SAKSSYOH BY FURNACE FOR SINTERING POWDER RELATED APPLICATIONS This application is a division of application, Ser. No. 12,701, filed Feb. 19, 1970, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of sintering powder bodies and a furnace for carrying out the method.

2. The Prior Art High density and freedom from pores in hard metal bodies metallurgically manufactured from powder for tools give high quality. For cutting tools the high density gives increased wear-strength and less risk of the edge breaking. For rollers and the like the freedom from pores gives increased strength and a smoother surface, which also means that a product being rolled can be given a smoother surface. Even in the manufacture of electric resistance bodies, of for example MoSi a very high density and freedom from pores provides important advantages. The strength is increased and the risk of local over-heating and resultant burnout is decreased. The advantages with high density and freedom from pores are equally great for cermets of various types.

Great density and freedom from pores have previously been achieved by enclosing a pressed powder body in a gastight, heat-resistant sheath of some suitable metal, after which the sheath was evacuated, sealed and placed in a furnace in which the material was sintered under high pressure. Temperatures of up to 1500 C and pressures of up to 2000 bar were used. Placing a sheath around a powder body, particularly one of complicated shape, evacuating and sealing the sheath and then removing the sheath after the pressure sintering process, is extremely expensive. Particularly when manufacturing small cutting elements the sheathing involves considering expense. The problem is how to perform sintering and hot moulding under such conditions that the desired quality is obtained without the expensive sheathing.

SUMMARY OF THE INVENTION The present invention relates to a method of manufacturing sintered bodies from powdered material, the

method comprising sintering, preferably under vacuum, powdered bodies which have beenconventionally produced by compressing powder and then hot moulding the bodies after sintering under direct influence of a pressure medium. The sintering of the bodies and the subsequent compression are thus carried out without the bodies being encapsuled in a gastight sheath. According to the invention bodies made of powder or a powder mixture are sintered at at mospheric pressure or lower pressure and at such a temperature that powder grains of the same material are bound together or that a binder binds together other powder grains in the mixture, after which the bodies are isostatically hot pressed under direct influence of a pressure medium such as argon, helium, nitrogen or hydrogen. A possible explanation of the fact that the density can be increased by isostatic hot pressing of sintered products without the bodies being sure and temperature are selected so that the binder is sufficiently deformable for powder grains held together to move closer together and fill the empty spaces.

A special furnace is used for the process, of the type having an insulated furnace chamber enclosed in a pressure chamber. The pressing process is carried out immediately after the sintering and is started at the sintering temperature. During the pressing the temperature must be set at at least such a value that the binder is sufficiently deformable for the sealed spaces to be compressed. The temperature and pressure are dependent on each other and on the powder composition.

The process is applicable to powders of many different types, particularly hard metal products containing WC, TaC, TiC or VC or a mixture of two or more of these substances and a binder consisting of Co and/or Ni and possibly Fe. Bodies of powder are first conventionally sintered at atmospheric pressure or lower pressure, preferably under vacuum so that the carbide grains of the substances are combined. The quantity of binder and the temperature are selected so that most of the remaining spaces or pores between the powder grains will be completely sealed and the sintered body 'is then isostatically compressed in a furnace having a furnace chamber enclosed in a pressure chamber. This compression is carried out advantageously at the sintering temperature, Le. a temperature at which the binder is at least relatively easily plastically deformed or possibly liquid or at least nearly liquid, and with the pressure medium acting directly on the surface of the body. Thus the expensive sheathing of the, powder bodies previously considered necessary for pressure sintering or isostatic hot pressing is avoided. The hard metal bodies usually contain 0 99 WC and/or TiC, 0 30 TaC, NbC and/or VC as hard particles and 1 30 Co and/or Ni and possibly iron as binder. Most usually the bodies contain 0 99 WC and/or TiC, l

30 Co and/or Ni and other substances in small quantities, mostly in the form of impurities. The actual sintering process may normally take place at a somewhat lower temperature than for conventional sintering at atmospheric pressure or under vacuum, since it is unnecessary to fill the spaces between the grains as it was previously during the sintering process. It is important only that spaces and pores are well sealed so that isostatic compression without a special sheath can be satisfactorily carried out. For the same reason a slightly lower percentage of binder may possibly be used. Lower temperature also means that there is less risk of the binder running down in the body during the sintering process. It may even be sufficient only to sinter the powder body on the surface prior to the isostatic compression. The sintering temperature is usually 1200 1600 C. The subsequent isostatic compression is in most cases carried out at 800 1500 C and 100 2000 bar. As a general rule, increased pressure means that a lower temperature can be used to achieve the same density. The sintering and the subsequent compression give the best result if carried out in one and the same furnace. If the sintering is carried out under vacuum a pressure chamber is required which has a separate vacuum valve having large flow cross-section in order to achieve the required pressure drop. The grain size of the powders preferably used is generally I 8M.

The method is also suitable for the manufacture of electrical heating bodies of, for example MoSi and glass. Bodies made of powder are sintered at a temperature of 900 l400 C, after which they are isostatically compressed under direct influence of a pressure medium at 100 2000 bar and 800 1400 C. The grain size of the powder used is generallyl 7511..

A third group of products for which the-method can be used is cermets of various types. A cermet comprising 90 99 M and 1 l0 Ag is manufactured by compressing a powder having a grain size of l 50p to bodies which are sintered under vacuum, usually at l0 torr or lower and at 900 l400 C,- so that sealed pores or spaces are obtained, after which the bodies are isostatically hot pressed while in direct contact with a pressure medium at 100 2000 bar and 800 [400 C.

A fourth product which can be produced is pellets of uranium dioxide. Powder of uranium dioxide only is sintered in a hydrogen gas atmosphere or under vacuum at a temperature of at least lS50 C and then hot pressed at a pressure of I00 2000 bar and a temperature of 1 300 l600 C.

As an'example of the technical progress this method involves it may be mentioned that sintered bodies manufactured according to conventional methods usually have I00 500 times as many spaces per volume unit as bodies manufactured in accordance with the method according to the-invention. In comparison with previous methods of pressure sintering bodies of powder enclosed in an evacuated heat-resistance sheath, the advantage is also gained that a time consuming and expensive working operation is eliminated. The method makes possible sintering without asheath, without cooling the bodies between sintering and hot pressing and without complicated transport means with sluices to prevent the hot bodies from coming into contact with air. In the pressing of uranium dioxide U0 a density of up to 99.5 percent of the density which could in theory be achieved has been obtained.

The invention also relates to a furnace in which both sintering and hot pressing can be carried out and which is therefore particularly suitable for carrying out the method. The furnace according to the invention is of the type comprising a cylindrical pressure chamber having a high pressure cylinder, end closures projecting into this, and means for taking up axial forces effected by a pressure medium on the end closures. The furnace chamber and heating means of the furnace are surrounded by an insulating sleeve with insulating lid and bottom. ltis characterized in that one end closure of the pressure chamber, usually the lower one, is shaped with an annular part having members sealingagainst the cylinder or via an intermediate element, a valve plate covering the opening of the annular part, sealing members existing between the annular part and the valve plate, a vacuum conduit which can be sealingly connected to the annular part of the end closure, and also means to separate the valve plate and the annular part of the closure so that an opening having relatively large flow area can be achieved for vacuum sintering.

Usually the vacuum conduit is connected to the furnace in such a way that it is pressed into the annular opening of the end closure. The connection conduit can be used to lift the valve plate in relation to the annular part. In order to prevent the charge in the furnace from being moved or shaken the conduit is inserted until it comes into contact with the valve plate, after which the annular part of the end closure is lowered so that an opening is obtained between the furnace chamber and the vacuum pump. In order to reduce the stresses in the valve plate, particularly if this has large diameter and little thickness, it may be provided with a projecting part which, when the valve opening is closed, rests against the member which takes up the forces operating on the end closure.

BRIEF DESCRIPTION OF THE DRAWINGS The furnace according to the invention is described in more detail with reference to the accompanying drawings.

FIG. 1 shows a furnace, partly in section, with a transportable press stand which is pushed in over a high pressure chamber having a furnace chamber and takes up forces operating on the end closures when the furnace is placed under pressure.

FIGS. 2 and 3 show a section through the lower part of the pressure chamber during hot pressing and vacuum sintering, respectively,

FIG. 4 a section through the upper part of the pressure chamber and FIGS. 5 and 6 sections through the end closure taken at A-Aand B-B, respectively, in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings 1 designates a press stand which is movable between the position shown in the drawingand a position where it surrounds the high pressure chamber 2. The press stand is of the type consisting of yokes 3 and 4, spacers 5 and a strip sheath 6. The stand is supported by wheels 7 running on rails 8. The high pressure chamber 2 is supported by a pillar 9. This contains a high pressure cylinder consisting of an inner tube 10 and a surrounding strip sheath 1 l as well as end rings 12 which keep the strip sheath together axially and provide grips for brackets by which the high pressure chamber is attached to the pillar 9. The chamber 2 has an upper end closure 13 which projects into the tube 10 of the high pressure chamber. Between the tube and the end closure is a sealing ring 14. The furnace chamber 15 in which the charge 16 is inserted is surrounded by an insulating sleeve 17 consisting of three concentrical metal tubes 18, 19 and 20, layers 21 and 22 of insulating material and two end rings 23 and 24. The sleeve 17 is suspended by the ring 23 in the upper end closure 13 and is gastightly connected to this. In the upper part of the sleeve 17 is an insulating lid 25 consisting of a metal sleeve 26 filled with insulating material 27 and provided with bushings 28 for conductors to heating elements 47. The sleeve 26 is attached to a ring 29 clamped between the ring 24 and the upper end closure 13. There are grooves in the ring 29 for sealing rings 30 and 31. Between the lid 25 and the upper end closure 13 a closed space 32 is formed which communicates with the furnace chamber only through a pressure equalizing opening 33. In the space 32 a heating element can be joined with a feeding conductor, but this is not shown in the drawings. In the end closure is a through channel 34 with sealing members 36 for a feeding conductor 35. There is also a channel 37 to supply pressure medium and a groove for the sealing ring 14. The channel 37 opens into the gap 39 between the inner wall of the tube 10 and the outer tube 18 in the sleeve 17. The heating element 47 is supported by a tube 40 which surrounds the furnace chamber 15. The elements are attached to the outside of the tube 40 with the help of bars 41 having slots through which the heating elements can pass. The tube 40 is suspended in the lid by means of protective bars 43 on the inside of the tube. Between the lid 25 and the tube is a gap 44 so that openings are formed through the connection parts of the heating elements can pass from the gap 46 between the tube 20 of the sleeve 17 and the tube 40. Above the upper end closure 13 is a pressure plate having grooves for conductors 35.

The pressure chamber has a lower end closure consisting of an annular part 50 with a sealing 51 sealing against the inner surface of the tube 10. The annular part 50 has a flat valve seat 52 which is formed between the smaller recess 53 at the outer part and the large recess 54 at the inner part. In the annular part 50 is a valve plate 55 with a seal 56 which seals against the valve seat surface 52. The valve plate 55 is shaped with a downwardly projecting peg 57 having such a length that its lower surface is in the same plane as the lower surface of the ring 50 when the valve is closed and thus rests against the yoke 4 of the stand when the pressure forces the end closure against this. The valve plate 55 is provided with a number of axial guides 58 which are guided by the recess 54 and thus center the valve plate 55 in relation to the annular part 50. On the valve plate 55 rests an insulating bottom projecting into the insulating sleeve 17 and consisting of a metal ring attached to the valve plate and having a thick layer of insulating material 61. On this rests a plate 62 on which the charge 16 is placed. This plate may suitably be made of a material having good heat conductivity and may be provided with radial channels 63 and 64 between the periphery and a central space 65. Since the plate 62 has good heat conductivity the central part of the billet 16 can receive heat by way of the plate. The channels help gas to circulate between the space 65 and the gap around the plate so that heat can be transported to or from the central part of the plate.

In order to generate a vacuum in the furnace there is a vacuum pump equipment, not shown, which is connectable to the pressure chamber by means of the lower end closure shaped as a valve. The connection is done by means of a suction tube which projects into the recess 53 of the annular part 50 and is sealed to this by a sealing ring 71. The outermost part of the tube 70 has slits 72 between which the remaining material forms projecting supports 73. In the embodiment shown in the drawings the suction tube is projected into the recess 53 so that the supports 73 come into contact with the valve plate 55, after which the annular part is lowered to the position shown in FIGS. 1 and 3 and the valve is opened. There now is free communication between the furnace chamber in the pressure chamber and the section tube through the gap between the valve plate, the recess 54 of the annular part 50 and the valve seats 52 and the gaps 62 between the suction tube supports 63. The valve can-also be opened by letting the suction tube lift the valve plate.

The annular part 50 is attached by brackets to a casing 81 running along a guide 82 attached to the pillar 9. The casing, and thus the annular part 50 with the valve plate 55 and the bottom placed on this, and the charge can be raised and lowered by an operating cylinder 83, the operating rod 84 of which is attached at brackets 85 on the casing 81.

The equipment operates in the following way: The furnace is charged with material 16 with the press stand positioned in relation to the high pressure cylinder as shown in FIG. 1. The charge is placed on the furnace bottom when the lower end closure is in its lowered position, after which the operating cylinder 83 lifts the entire end closure with the charge to an upper position, see FIG. 2. The suction tube 70 is then inserted in the opening in the end closure, after which the annular part 50 is lowered to the position shown in FIG. 3. The valve plate 55 with the charge 16 will then be supported by the supports 73 of the suction tube 70 and a gap is formed between the valve plate 55 and the annular part 50. The charge is vacuum sintered, after which the annular part 50 is lifted and the suction tube 70 removed. The press stand 1 is then pushed in over the pressure chamber. The pressure chamber is supplied with pressure medium and sintered material is hot pressed to the desired density.

The invention is of course not limited to the embodiment shown and described. Many variations are possible within the scope of the following claims.

I claim:

'1. Furnace for performing vacuum sintering and isostatic hot pressing of powder bodies, comprising a cylindrical pressure chamber (2) having a high pressure cylinder (10,11,12), end closures (13,50,55) projecting into said cylinder, means (1) to taking up axial forces effected by a pressure medium on the end closures (13,50,55), a furnace chamber (15) arranged in the pressure chamber and having heating means (47), insulation surrounding the furnace chamber in the form of a sleeve (17) having an insulating lid (25) and bottom (60,61,62), in which one end closure (50, 55) of the pressure chamber is shaped with an annular part (50) having members (51) sealing against the cylinder (10,11,12) and having a valve plate (55) covering the opening of the annular part (50), sealing members (56) between the annular part (50) and the valve plate (55), a vacuum conduit (70) which can be sealingly connected to the annular part (50) of the end closure, and means to separate the valve plate (55) and the annular part (50) of the end closure.

2. Furnace according to claim 1, in which in that the connecting conduit (70) for the vacuum projects into the annular opening of the end closure and constitutes the means to separate the valve plate (55) from the annular part (50) of the end closure.

3. Furnace according to claim 2, in which the annular part (50) of the end closure can be lowered in order to separate this part (50) from the valve plate (55).

4. Furnace according to claim 1, in which the valve plate (55) is provided with a central projecting part (57 which during the hot pressing process rests against the member (1) which takes up the axial forces operating on the end closure. 

1. Furnace for performing vacuum sintering and isostatic hot pressing of powder bodies, comprising a cylindrical pressure chamber (2) having a high pressure cylinder (10,11,12), end closures (13,50,55) projecting into said cylinder, means (1) to taking up axial forces effected by a pressure medium on the end closures (13,50,55), a furnace chamber (15) arranged in the pressure chamber and having heating means (47), insulation surrounding the furnace chamber in the form of a sleeve (17) having an insulating lid (25) and bottom (60,61,62), in which one end closure (50, 55) of the pressure chamber is shaped with an annular part (50) having members (51) sealing against the cylinder (10,11,12) and having a valve plate (55) covering the opening of the annular part (50), sealing members (56) between the annular part (50) and the valve plate (55), a vacuum conduit (70) which can be sealingly connected to the annular part (50) of the end closure, and means to separate the valve plate (55) and the annular part (50) of the end closure.
 2. Furnace according to claim 1, in which in that the connecting conduit (70) for the vacuum projects into the annular opening of the end closure and constitutes the means to separate the valve plate (55) from the annular part (50) of the end closure.
 3. Furnace according to claim 2, in which the annular part (50) of the end closure can be lowered in order to separate this part (50) from the valve plate (55). 